BEIRUT — A rebel force of mostly Islamist fighters overran the largest military air base in northern Syria on Friday, striking a major blow against the government of President Bashar al-Assad and demonstrating the rising strength of religious extremists within the Syrian opposition.

Among the fighters who participated in the 10-day battle to take the Taftanaz base in Idlib province was the al-Nusra Front, a militant group that is thought to have links to al-Qaeda and has been designated a foreign terrorist organization by the U.S. government.

The Syrian military bombed the air base repeatedly Friday afternoon, but rebel fighters said they were still in control.

“It’s a very big victory for the Islamic groups,” said the director of the Britain-based Syrian Observatory for Human Rights, who uses the pseudonym Rami Abdulrahman. “They are the strongest in Syria now because the Western countries won’t help.”

The rebel victory came as the joint U.N. and Arab League envoy for Syria, Lakhdar Brahimi, met Friday with U.S. and Russian officials in Geneva, apparently inconclusively. Hopes of a political resolution to the Syria conflict dimmed Sunday after Assad gave a speech ruling out talks with the opposition. Brahimi’s role as a mediator is being questioned by Syria’s Foreign Ministry, which issued a statement Thursday calling him “flagrantly biased.”

Taking control of the Taftanaz air base may be the biggest strategic victory yet for rebel fighters. The bloody conflict in Syria has dragged on for nearly two years, leaving more than 60,000 dead, according to U.N. estimates. Rebels have demonstrated a greater degree of coordination and skill on the battlefield in recent months, taking over more than half a dozen military bases since the beginning of December and expanding their arsenal with confiscated weapons.

An activist who accompanied rebel fighters said the Taftanaz base was targeted because helicopters from there have frequently attacked civilians. “The helicopters of the regime used to take off from Taftanaz airport to bomb all the towns and cities in Idlib,” said the activist, who goes by the name Fadi Yaseen.

Approximately 1,000 rebel fighters attacked the base, where about 400 Syrian military personnel were based, Yaseen said. He estimated that 100 Syrian troops and 20 rebels were killed during the fighting.

A video posted online Friday shows the bloodied bodies of several Syrian military personnel, whom the cameraman identifies as pilots. They appear to have been shot en masse in a shallow hole. “May Allah punish you for what you did,” a man says off-camera.

In a reminder of how anti-government groups continue to use social media to spread the word about their progress in the conflict, one fighter or activist posted a live, 34-minute video stream on the site bambuser.com as he walked around the base Friday afternoon. At one point, the cameraman stopped and focused on half a dozen dead Syrian troops slumped over in bloody uniforms.

“We called on them to defect for the past 10 days, but they refused,” the man says on the video. “They died for their refusal.”

Some of those who helped capture Taftanaz were members of the rebel Free Syrian Army, Yaseen said. But he and Abdulrahman said the bulk of the attacking force was made up of fighters from the al-Nusra Front, along with Ahrar al-Sham and Talia al-Islamiya, two other religious extremist groups.

Timeline: Major events in Syria’s tumultuous uprising that began in March 2011.

A video posted online Friday shows a fighter gleefully raiding a weapons storage facility while a group of fellow fighters bow down and pray nearby. Another video shows a couple of fighters standing on a helicopter at the base and chanting, “We want an Islamic state.”

Some observers have expressed concern that the success of the religious extremists will lead to more sectarian violence against members of Assad’s minority Alawite sect. Under Assad’s authoritarian rule, Syria’s government has been steadfastly secular. Many of the Islamists fighting against the government view anyone who does not follow their version of hard-line Sunni Islam as an infidel.

In a video posted online last week, more than two dozen fighters from the al-Nusra Front stand in front of a black flag with Koranic scripture, brandishing machine guns and rocket-propelled grenades.

“O, you nusereyeen and your troops,” the speaker says, using a derogatory reference for followers of the Alawite faith. “We dedicate ourselves only to battle you and exterminate you.”

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